Capacitor drop type power supply circuits are commonly used in domestic appliances for converting AC mains power to DC supply for the appliance. The AC mains supply may first have its voltage lowered by a transformer. FIG. 1 illustrates a typical example of a capacitor drop power supply circuit. The circuit comprises a first capacitor 4A, a resistor 3A and a diode 5A. A zener diode 7A is provided as a voltage reference. A second capacitor 9A in series with a diode 8A is located in parallel with the zener diode 7A. During a positive half cycle of the input voltage current flows in the direction I+ through the load and to charge up second capacitor 9A. During the negative half cycle current flows in the opposite direction I− through the resistor 3A, capacitor 4A and diode 5A. A load current IL is supplied via discharge of second capacitor 9A.
The input impedance of this power supply circuit is mainly dependent upon the value of the drop capacitor 4A and does not change greatly regardless of the load on the power supply. In the case a domestic appliance's load varies between normal operation mode and standby mode. Many appliances are not turned off and spend most of their time in standby mode when the actual current draw of the device is very low. However, because the input impedance of the power supply circuit is mainly dictated by capacitor 4A, the power drawn by the power supply is relatively constant regardless of whether the appliance is in normal operation mode or standby mode.